Cancer is caused by abnormalities in a cell’s DNA (its genetic blueprint).
Abnormalities may be inherited from parents, or they may be caused by outside
exposures to the body such as chemicals, radiation, or even infectious agents.
Some carcinogens do not act on DNA directly, but cause cancer in other ways,
such as causing cells to divide at a faster rate. Substances that can cause
changes that can lead to cancer are called carcinogens.

Carcinogens do not cause cancer in every case, all the time. Substances
classified as carcinogens may have different levels of cancer-causing potential.
Some may cause cancer only after prolonged, high levels of exposure. And for any
particular person, the risk of developing cancer will depend on many factors,
including the length and intensity of exposure to the carcinogen and the
person’s genetic makeup.

How Do We Determine if Something Is a Carcinogen?

Scientists get much of their data about whether or not something might be
carcinogenic from laboratory (cell culture and animal) studies. Although it
isn’t possible to predict with absolute certainty which substances will be
carcinogenic to humans based on animal studies alone, virtually all known human
carcinogens that have been adequately tested in lab animals produce cancer in
these animals. In many cases, carcinogens are first found to cause cancer in lab
animals and are later found to cause cancer in people. Because there are far too
many substances (natural and manmade) to test each one in lab animals,
scientists use knowledge about chemical structure, other types of lab tests, and
information about the extent of human exposure to select chemicals for testing.

Most studies of potential carcinogens in lab animals expose the animals to
doses that are higher than common human exposures. This is so that cancer risk
can be detected in relatively small groups of animals. For most carcinogens, it
is assumed that those that cause cancer at larger doses in animals will also
cause cancer in people. Although it isn’t always possible to know the
relationship between exposure dose and risk, it is reasonable for public health
purposes to assume that lowering human exposure will reduce risk.

Another important way to identify carcinogens is through epidemiologic
studies, which look at the factors that might affect the occurrence of
cancer in human populations. While these studies also provide useful
information, they also have their limitations. Humans do not live in a
controlled environment. People are exposed to numerous substances at any one
time, including those they encounter at work, school, or home; in the food they
eat; and the air they breathe. And it is usually many years (often decades)
between exposure to a carcinogen and the development of cancer. Therefore, it
can be very difficult to single out any particular exposure as having a definite
link to cancer.

By combining data from both types of studies, scientists are able to make an
educated assessment of a substance’s cancer-causing ability. When the available
evidence is compelling but not felt to be conclusive, the substance may be
considered to be a probable carcinogen.

How Are Carcinogens Classified?

The most widely used system for classifying carcinogens comes from the
International Agency for Research on Cancer (IARC), which is part if the World
Health Organization (WHO). The IARC has evaluated the cancer-causing potential
of about 900 likely candidates in the last 30 years, placing them into one of
the following groups:

Group 1: Carcinogenic to humans

Group 2A: Probably carcinogenic to humans

Group 2B: Possibly carcinogenic to humans

Group 3: Unclassifiable as to carcinogenicity in humans

Group 4: Probably not carcinogenic to humans

Perhaps not surprisingly, most of the agents are of probable, possible, or
unknown risk. Only about 90 are classified as "carcinogenic to humans."

In the United States, the National Toxicology Program (NTP) releases the
Report on Carcinogens every two years. The NTP is formed from parts of
several different government agencies, including the National Institutes of
Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food
and Drug Administration (FDA).

The Report on Carcinogens (RoC) identifies two groups of agents:

"Known to be human carcinogens"

"Reasonably anticipated to be human carcinogens"

Unlike the IARC’s list, the RoC does not list substances that have
been studied and found not to be carcinogens.

Below are the lists of known and probable human carcinogens from both groups.

Some Important Points to Know About These Lists

The IARC and NTP act independently but have studied many of the same
agents; therefore many known or suspected carcinogens appear on both lists.
But because an agent appears on one and not on the other does not
necessarily mean there is a controversy, as one agency may not have
evaluated it.

These lists are not necessarily all-inclusive - they include only those
substances that have been evaluated by the agencies.

Most of the agents on the list are connected only with certain kinds of
cancer, not all types. For more detailed information, refer to the specific
monographs or reports published by the agencies (available on their
respective Web sites).

The lists themselves say nothing about the relative carcinogenicity of
the agents. Carcinogens do not cause cancer at all times, under all
circumstances. Some may only be carcinogenic if a person is exposed in a
certain way (for example, ingesting as opposed to touching). Some of these
agents may lead to cancer after only a very small exposure, while others
might require intense exposure over many years. Again, you should refer to
the agencies’ reports for specifics.

It is also important to keep in mind that not all carcinogens are to be
avoided at all costs. The lists include many commonly used medicines,
particularly some hormones and drugs used to treat cancer. Tamoxifen, for
example, increases the risk of certain kinds of uterine cancer, but
decreases the risk of recurrence (return) of breast cancer, which may be
more important for some women. If you have questions about a medicine you
are taking that appears on one of these lists, be sure to ask your doctor.

Known Human Carcinogens

International Agency for Research on Cancer (IARC)

"Carcinogenic to Humans" (Group 1)

Agents and Groups of Agents
Aflatoxins (naturally occurring mixtures of)
4-Aminobiphenyl
Arsenic and arsenic compounds
(Note: This evaluation applies to the group of compounds as a whole
and not necessarily to all individual compounds within the group)
Asbestos
Azathioprine
Benzene
Benzidine
Beryllium and beryllium compounds
N,N-Bis(2-chloroethyl)-2-naphthylamine (Chlornaphazine)
Bis(chloromethyl)ether and chloromethyl methyl ether (technical-grade)
1,4-Butanediol dimethanesulfonate (Busulphan; Myleran)
Cadmium and cadmium compounds
Chlorambucil
1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (Methyl-CCNU;
Semustine)
Chromium [VI] compounds
Cyclophosphamide
Cyclosporin
Diethylstilbestrol
Epstein-Barr virus
Erionite
Estrogen therapy, postmenopausal
Estrogens, nonsteroidal
(Note: This evaluation applies to the group of compounds as a whole
and not necessarily to all individual compounds within the group)
Estrogens, steroidal
(Note: This evaluation applies to the group of compounds as a whole
and not necessarily to all individual compounds within the group)
Ethylene oxide
Etoposide in combination with cisplatin and bleomycin
[Gamma Radiation: see X- and Gamma (g)-Radiation]
Helicobacter pylori (infection with)
Hepatitis B virus (chronic infection with)
Hepatitis C virus (chronic infection with)
Herbal remedies containing plant species of the genus AristolochiaHuman immunodeficiency virus type 1 (infection with)
Human papillomavirus type 16
Human papillomavirus type 18
Human T-cell lymphotropic virus type I
Melphalan
8-Methoxypsoralen (Methoxsalen) plus ultraviolet A radiation
MOPP and other combined chemotherapy including alkylating agents
Mustard gas (Sulfur mustard)
2-Naphthylamine
Neutrons
Nickel compounds
Opisthorchis viverrini (infection with)
Oral contraceptives, combined
(Note: There is also conclusive evidence that these agents have a
protective effect against cancers of the ovary and endometrium)
Oral contraceptives, sequential
Phosphorus-32, as phosphate
Plutonium-239 and its decay products (may contain plutonium-240 and other
isotopes), as aerosols
Radioiodines, short-lived isotopes, including iodine-131, from atomic reactor
accidents and nuclear weapons detonation (exposure during childhood)
Radionuclides, alpha-particle-emitting, internally deposited
(Note: Specific radionuclides for which there is sufficient
evidence for carcinogenicity to humans are also listed individually as Group 1
agents)
Radionuclides, beta-particle-emitting, internally deposited
(Note: Specific radionuclides for which there is sufficient
evidence for carcinogenicity to humans are also listed individually as Group 1
agents)
Radium-224 and its decay products
Radium-226 and its decay products
Radium-228 and its decay products
Radon-222 and its decay products
Schistosoma haematobium (infection with)
Silica, crystalline (inhaled in the form of quartz or cristobalite from
occupational sources)
Solar radiation
Talc containing asbestiform fibers
Tamoxifen
(Note: There is also conclusive evidence that this agent (tamoxifen)
reduces the risk of contralateral breast cancer)
2,3,7,8-Tetrachlorodibenzo-para-dioxin
Thiotepa
Thorium-232 and its decay products, administered intravenously as a colloidal
dispersion of thorium-232 dioxide
Treosulfan
Vinyl chloride
X- and Gamma (g)-Radiation

10th Report On Carcinogens. U.S. Department of Health and Human Services.
Public Health Service - National Toxicology Program. 2002. Available online at
http://ehp.niehs.nih.gov/roc/toc10.html. Accessed February 2003.